Apoptosis is a morphologically distinct from of programmed cell death that plays important roles in development, tissue homoestasis and a wide variety of diseases, including cancer, AIDS, stroke, myopathies and various neurodegenerative disorders. It is now clear that apoptosis occurs by activating an intrinsic cell suicide program which is constitutively expressed in most animal cells, and that key components of this program have been conserved in evolution from worms to insects to man. A central step in the execution of apoptosis is the activation of an unusual class of cysteine proteases, termed caspases, that are widely expressed as inactive zymogens. The overall objective of the proposed research is to gain insight into the molecular mechanisms that control caspase activation and cell death. The specific goals of this proposal are to characterize wild type and mutant versions of genes that were originally identified in genetic screens for cell death modifiers in Drosophila. We will use a multidisciplinary approach that integrates genetics, biochemistry, cell biology, Drosophila gene transfer and mammalian cell transfection experiments to elucidate the mechanism by which the pro-apoptotic proteins REAPER, HID (head involution defective) and GRIM kill. In particular, we will study functional and biochemical interactions between REAPER, HID and GRIM with Drosophila and human inhibitor of apoptosis proteins (IAPs). We also propose to characterize several novel genes corresponding to mutants that strongly affect REAPER, HID and GRIM-induced cell death, and to define their precise role and mechanism of action during apoptosis. Finally, we will investigate to what extent the apoptotic pathway has been conserved between Drosophila and mammals. This work should significantly advance our understanding of how apoptosis is regulated, and how this process can be manipulated for therapeutic purposes.